This invention relates to the field of the preparation of p-aminophenol and, more particularly, to an improved process for the manufacture of p-aminophenol by catalytic hydrogenation.
Various processes are known to the art for the manufacture of p-aminophenol. In the process described in Benner U.S. Pat. No. 3,383,416, nitrobenzene is catalytically hydrogenated in the presence of a dilute sulfuric acid solution. Under the conditions utilized in the Benner patent, nitrobenzene is subject to complete reduction to aniline by reaction with three moles of hydrogen. ##STR1## However, the intermediate phenylhydroxylamine, produced by reaction of nitrobenzene with two moles of hydrogen, is rapidly removed from the reaction zone by absorption into the sulfuric acid phase where it rearranges to p-aminophenol: ##STR2## Fortuitously, the noble metal catalyst tends to congregate in the nitrobenzene phase so that phenylhydroxylamine absorbed into the acid phase is essentially removed from the hydrogenation reaction zone, and conversion of phenylhydroxylamine to aniline is thereby inhibited. This feature also permits recovery of the catalyst in the nitrobenzene phase when the phases are permitted to separate so that, by terminating the reaction prior to the conversion of all the nitrobenzene, the catalyst is easily recovered by decantation and recycled in its nitrobenzene vehicle for use in another batch, or in an earlier stage of a cascade continuous hydrogenator.
Despite its numerous salutary features, the Benner process does not entirely avoid conversion of phenylhydroxylamine to aniline. Thus, typically the yield of p-aminophenol is about 75% while 15% of the nitrobenzene is reduced to aniline. The amount of by-product aniline obtained can be minimized by lowering the reaction rate, but this, of course, involves a sacrifice in productivity.
Rylander et al. U.S. Pat. No. 3,715,397 discloses a process for preparation of p-aminophenol by catalytic hyrogenation of nitrobenzene using a platinum oxide catalyst in the presence of sulfuric acid and dimethyl sulfoxide. Phenylhydroxylamine produced in the hydrogenation is absorbed into the acid phase and converted in situ to p-aminophenol. The acid strength is that obtained by mixing 25 ml. concentrated sulfuric acid with 75 ml. water. Reaction is carried out in a batch system until absorption of two moles of hydrogen per mole of nitrobenzene. According to the Rylander et al. disclosure, the reaction may be carried out over a wide range of temperatures, with room temperature apparently being preferred.
In the co-pending and co-assigned application of Thomas H. McFadden, Ser. No. 343,997, a process is described in which dimethyl sulfoxide or another organic sulfoxide is incorporated into a continuous process for the manufacture of p-aminophenol by catalytic hydrogenation of nitrobenzene. In the McFadden process, 50 to 90 percent of the nitrobenzene charged to the first of three cascade hydrogenators is reduced leaving an unreacted, nitrobenzene fraction which is allowed to separate from the reaction mixture and in which the catalyst congregates. After separation, the nitrobenzene phase containing the catalyst is recycled to the first hydrogenator.